Feature: add checkbox for default value of 'Squash commits when merge request is accepted'

What does this MR do?

This MR adds a project setting Enable 'Squash commits when merge request is accepted' option by default, similarly to the new Enable 'Delete source branch' option by default project setting (366b55f4).

Such a default value is very useful since it is easy to forget to enable that checkbox, which results in unnecessary commits to be merged.

I don't have much experience with Ruby and no experience with the GitLab codebase, so I hope I implemented this correctly and didn't do too bad things :-)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

The risk should be pretty low, since this doesn't change any default.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done